• Quick retrieval from earlier learning (Orbits and orbital speed (v = 2πr / T); Force, mass and acceleration (F = ma)). Students answer on mini whiteboards.
• Show answers together and note who needs support in the main task.

1. The International Space Station orbits at a radius of about 6780 km from Earth's centre, taking 92 minutes per orbit. What is its orbital speed, in km/s? (v = 2πr ÷ T; give 1 decimal place) → 7.7 km/sPrompt: Convert the period to seconds.
2. What resultant force is needed to accelerate a 80 kg mass at 0.5 m/s²? → 40 NPrompt: F = m × a
3. The Moon takes about 27 days to orbit Earth, but the ISS takes about 90 minutes. Why the difference? (The ISS orbits much closer, so it must move faster around a much shorter path / The Moon is pushed by the Sun / The ISS has no mass / The Moon orbits a different planet) → The ISS orbits much closer, so it must move faster around a much shorter pathPrompt: Compare their distances from Earth.

• Drop a sheet of paper and a scrunched-up sheet together and ask why one falls faster.
• Share the objective: Circular motion and centripetal force. Read the success criteria together.

• Work through the example out loud, saying why you do each step, not just what you do.
• Example: A 1 kg object moves in a circle of radius 2 m at 5 m/s. What centripetal force is needed, in N? (F = m v² ÷ r)
• Step 1: 1 × 5² ÷ 2 = 12.5
• Step 2: So the answer is: 12.5 N
• Answer: 12.5 N
• Pause after each step and ask a student to say what comes next.

• Do these together. Students try each step on whiteboards before you reveal it.
• Use the hints as prompts rather than giving the answer.

1. A 2 kg object moves in a circle of radius 1 m at 10 m/s. What centripetal force is needed, in N? (F = m v² ÷ r) → 200 NPrompt: Square the speed first.
2. Friction between the tyres and the road provides 3200 N of centripetal force for a 1000 kg car on a bend of radius 20 m. What is the car's speed, in m/s? → 8 m/sPrompt: Rearrange: v² = F r ÷ m.

• Students work on the student sheet. Circulate and check the first two answers of every student early.
• Students who finish: write a question of their own for a partner, with the answer.

The 8 questions are on the student sheet below, with answers in the key.

• Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.

1. A car goes round a bend twice as fast. How does the centripetal force needed change? (it doubles / it becomes four times larger / it halves / it stays the same) → it becomes four times largerPrompt: F depends on v².
2. A 2 kg object moves in a circle of radius 10 m at 20 m/s. What centripetal force is needed, in N? (F = m v² ÷ r) → 80 NPrompt: Square the speed first.

Free online practice with hints and printable worksheets for this skill: eurekami.com/science/grade-12.